Such a hydraulic machine is known from EP 1 508 361 A1. This hydraulic machine is a pressure exchanger.
The invention is described in the following in connection with a hydraulic pressure exchanger as an example for a hydraulic machine. The invention can, however, be used in other kinds of hydraulic machines having working cylinders and kidney-shaped openings in a pressure shoe.
The operation of a pressure exchanger can briefly be summarized as follows: a working cylinder passing a first kidney-shaped opening in the pressure shoe connected to a high pressure supply port is filled with liquid under high pressure. Usually a working piston is arranged in the working cylinder. This working piston is moved by the incoming liquid in a direction away from the pressure shoe. The working cylinder on the other side of the working piston has previously been filled with fresh liquid. This fresh liquid is outputted under the action of the working piston. When the drum continues rotating, the working cylinder passes another kidney-shaped opening on the other side connected to a low pressure supply port so that the working cylinder is filled with fresh liquid pushing the working piston in opposite direction and outputs the liquid to be wasted.
A problem arises when the front opening of a working cylinder under high pressure leaves a kidney-shaped opening and passes a closed area on the pressure shoe. In this situation the pressure in the working cylinder creates forces acting against the pressure shoe. These forces tend to separate the pressure shoe and the front opening of the working cylinder. Such separation should be avoided since such separation leads to an unwanted internal leakage.
One approach for avoiding this internal leakage is to press the pressure shoe and the drum together with high forces. However, these high forces increase friction between the drum and the pressure shoe causing wear and noises which should be avoided as well.